Drills, generally of the portable type, are known wherein the output spindle on which the chuck is mounted performs a rotary movement as well as an axial reciprocating movement. The percussion effect resulting from such an axial reciprocating movement provides advantages when perforating materials having a tendency to crumble as opposed to materials which can be drilled by conventional methods involving the removal of chips in the course of the cutting action. Concrete stone, and the like are materials which tend to crumble.
As a rule, the axial reciprocating movement is brought about through the interaction of two sets of ratchet teeth shaped in the form of a cam, with one element being integral with the stationary portion of the drill while the other one is integral with the output spindle shaft. The axial pressure that is exerted by the operator onto the bit during the drilling operation causes the output spindle to bring the movable set of rachet teeth in contact with the stationary set of ratchet teeth. The overlapping of the respective sets of teeth of suitable profile results in a successive moving away of the output spindle shaft and the set of ratchet teeth mounted thereon. The respective sets of teeth are caused to reestablish contact through the pressure exerted by the operator on the drill so that the successive engagements of the teeth sets produces a beating action that is causing, in turn, the percussion of the output spindle and the chuck and tool bit mounted on the output spindle.
Such a mode of operation presupposes that the entire body of the drill constitutes the inertial reaction mass of the percussion effort of the output spindle, chuck and tool bit.
The shape of the stationary and movable ratchet teeth can generally be that of a sawtooth profile in which the inclined sections constitute the impact surfaces. The result is that the reaction on the stationary gear is not axial but has, on the contrary, an axial component and a tangential component on the plane perpendicular to the axis of the output spindle. Both of these components are rigidly transmitted to the body of the conventional hammer-drill.
It ought to be pointed out likewise that the above-mentioned conventional configuration of the percussion drill takes into account as a necessary consequence that the number of percussions per revolution of the output spindle be defined solely by the number of teeth of the stationary set of ratchet teeth and the rotating set of ratchet teeth. It follows from this premise that the percussion frequency is a linear function of direct proportionality to the speed of rotation of the output spindle which may not be desirable in all instances. Moreover, another inevitable consequence is that each percussion or impact blow may be in a well-defined angular position of the output spindle shaft; whereas, it would be advantageous to have a continuous variability so as to attack, in changing positions, the material subjected to the percussion force. By continously varying the angular positions at which the bit strikes the workpiece, such as concrete for example, a round bore is obtained rather than one which takes on the general contour of bit. This prevents the bit from binding in the workpiece.